Recently, we had a huge biology test and I spent so much time on studying to this test that I basically knew everything we were supposed to learn without any issues. My problem is to answer the questions that I am being asked correctly because I tend to write lesser than needed on biology tests. One question on the test was for example: What is aids? And I answered by writing about the HIV-virus, its functions (including GP120 and that it affects Th-cells and macrophages) and how it affects the body and the immunesystem. However my teacher added that I should had written about Th-cells' and macrophages' functions in the immunesystem as well and therefore I didn't get full points on that question and neither did I on the other questions on the test since this type of mistake was repeated througout the whole test.

So my question to you all is: How do you know what kind of answer they're looking for in a test without writing irrelevant facts?

Exam boards are notoriously fickle when it comes to marks. Sometimes the way you phrase something or the keywords you use will determine whether you get marks, rather than your understanding. This is often just a method of making marking easier.

In your example above, whilst you wrote about how HIV affects the body and its physiology, the question was about AIDS. It may have wanted you to elucidate exactly why the effects of an HIV infections could lead to the complete breakdown of immune function. For example if you had mentioned the T-helpers and macrophages you could have explained why these are essential for immune defence, and so why without them AIDS will happen.